Polychlorinated biphenyls (PCBs) represent a class of toxic xenobiotic chemicals that are distributed throughout the biosphere. Over the past several years, PCBs have received ever-increasing attention due to concerns about their toxicity and potential carcinogenicity. PCBs are produced by direct chlorination of biphenyl. Due to the large number of hydrogen atoms present on the biphenyl nucleus, many different chlorinated species (termed "congeners.revreaction.) are possible. As many as 210 congeners of the PCBs could be theoretically produced [Furukawa, Biodegradation and Detoxification of Environmental Pollutants, p. 44-57 (CRC Press 1982)]; however, due to steric restrictions, only about half this number are actually observed. Therefore, PCBs are mixtures of a variety of chlorine-substituted biphenyl molecules.
PCBs have been widely used industrially due largely to their thermal stability and flame retardance. Such characteristics encouraged PCB use in transformer oils and in other high-temperature applications. Until the early 1970's PCBs were found in various pesticide formulations. PCBs have also been used in plasticizers, heat transfer and capacitor systems, surface coatings, printing inks, carbonless duplication paper, and waxes. While industrial use of PCB has been sharply restricted significant quantities of PCBs are still being released into the environment from waste dumps and failures of old electrical equipment. PCB contamination has been observed in drinking water, wastewater, foods, and especially in fish.
PCBs are lipophilic. They accumulate and are bioenhanced in fatty tissues [Furukawa supra; Jacobson et al., Develop. Psychol., 20:523-532, (1984)]The physical effects of polychlorinated biphenyls vary from mammals, to birds, to humans. Mammals exposed to these chemicals exhibit marked changes in the liver [Fishbein, Ann. Rev. Pharmocol., 14:139-156, (1974)], including lesions, fatty infiltration, centrolobular atrophy necrosis, and liver cell enlargement. In the case of rats, hyaline degeneration, are evidence of such exposure. Among the adverse physical effects exhibited in birds, kidney damage, fluid around the heart, intestinal hemorrhage, and reduced spleen size, have been observed [See Rehfeld et al., Poultry Sci. 50:1090-1096, (1971)]. Dermatitis from surface exposure can occur in both mammals and birds [Voss et al., Toxicol. Appl. Pharmacol., 17:656-658 (1971)]. In addition to toxicity, PCBs may be carcinogenic and mutagenic (Fishbein, supra).
PCBs are not easily removed by natural microbial populations. In 1978, Furukawa et al., Applied Environ. Micro., 35:223-227 studied the biodegradability of several isomers of polychlorinated biphenyls. They found that as chlorine substitution increased, degradability decreased. An isomer with four or more chlorines was not easily degraded. The position of the chlorine was also important. Ortho positioning of two chlorines on a single ring greatly inhibited degradation. If all the chlorines were on the same ring, degradation occurred at a faster rate than for isomers with the same number of chlorines spread over both rings. It was noted that ring fission usually took place on the lesser or non-chlorinated ring. These results suggested that PCBs could be utilized by bacteria, but only in certain isomeric forms. The products of such degradation were usually chlorinated. Subsequent studies have verified these observations.
Considering the environmental importance of PCBs and the hazards they pose, numerous investigators have been examining biological detoxification systems to deal with PCBs. One way to decipher the complexities of highly chlorinated isomers is to look at lesser chlorinated isomers. One of these which has been studied is 4-chlorobiphenyl.
4-chlorobiphenyl is one of the three monochlorinated isomers that can result from the chlorination of biphenyl. As with the other biphenyls, it is very insoluble in water, but freely soluble in a variety of organic solvents. Pure 4-chlorobiphenyl is crystalline, white to off-white in color. It is classified as an irritant.
Unlike the highly chlorinated biphenyls, a number of microorganisms have been identified that can degrade 4-chlorobiphenyl. Wallnofer and Englehardt Chemosphere, 2:69-73 (1973) isolated a soil fungus, Rhizopus japonicus, which hydroxylated 4-chlorobiphenyl at the 4' position. Other intermediates were not noted. In 1973, Ahmed and Focht, Can. J. Microbiol, 19:47-52 discovered a species of Achromobacter which produced 4-chlorobenzoic acid from 4-chlorobiphenyl. A study by Masse et al. Appl. Environ. Micro., 47:947-951 (1984) also described the presence of 4-chlorobenzoate as a major metabolite of 4-chlorobiphenyl.
In 1982, Sylvestre and Fauteux J. Gen. Appl. Micro., 28:61-72, reported a facultative anaerobe able to utilize 4-chlorobiphenyl. Up until that time, only strict aerobes were reported to degrade the lesser chlorinated PCB isomers. This organism was tentatively identified as a member of the bacterial group IVe. As with other 4-chlorohiphenyl degraders, 4-chlorobenzoic acid accumulated in the culture media.
Sylvestre et al. Appl. Microbiol. Biotechnol., 21:192-195 (1985) reported that a two-membered bacterial culture was able to degrade both 4-chlorobiphenyl and 4-chlorobenzoate rapidly. An axenic culture was able to degrade 4-chlorobiphenyl alone, with a 45% decrease in substrate remaining after eight days. When incubated with a 4-chlorobenzoate degrader, 99% of the 4-chlorobiphenyl was degraded over the same time frame. However, no organism, to date, has been isolated which is able to grow on 4-chlorobiphenyl and degrade 4-chlorobiphenyl to 4'-chloroacetophenone and other metabolites.